Starting motors are commonly used to start engines by cranking them.
Starting motors may contain or develop defects. For example, a short may exist or develop in a winding of a starting motor. This can reduce the torque that the starting motor produces and cause the starting motor to draw excessive current. Mechanical problems may also develop, such as deterioration in a bushing. These mechanical problems can similarly cause a reduction in torque and excessive current to be drawn.
It may be difficult to detect defects in a starting motor, particularly without removing the starting motor from its installation. A reduction in torque or excessive current, for example, may not be apparent merely from observing and listening to the starting motor during operation. Even when it is, it may not be clear that the problem lies with the starting motor, as opposed to other components in the system (e.g., battery).
Current to the starting motor has in the past been measured as an indicia of its condition. However, this measurement often hinges upon the condition of its power source (e.g., battery), thus sometimes leading to ambiguous results.
Voltage to the starting motor has similarly been examined in the past as a means of assessing the condition of a starting motor. Again, however, this measurement often hinges on the condition of the power source (e.g., battery), again sometimes leading to ambiguous results. Further, a very powerful battery that is in good condition may mask a partially shorted starting motor from being detected by a voltage tester, as it may be able to sustain a high voltage level when driving the starting motor, even though the windings of the starting motor are partially shorted.